Tank and Pressure Vessel Skirt Thermal Ratcheting Prevention Device

ABSTRACT

A Tank and Pressure Vessel Skirt Ratchet Prevention Device for the reduction or elimination of relative deflection of a tank or vessel ( 29 ) skirt ( 25 ) is presented. In vertical tank or vessel ( 29 ) design where the tank shell ( 23 ) and the tank or vessel ( 29 ) bottom ( 22 ) are connected together by a knuckle ( 24 ) it is common to place a skirt ( 25 ) and skirt base plate ( 26 ) under the shell. The skirt ( 25 ) and skirt base plate ( 26 ) support the tank or vessel ( 29 ) structure, product and transmit these and environmental loads and anything on tank shell ( 23 ) into the foundation ( 21 ). Cyclic loads change the diameter of the skirt, ( 25 ) the tank shell ( 23 ) and skirt base plate ( 26 ). The addition of a ratchet prevention member ( 27 ) or an anti-friction device ( 30 ) reduces relative deflection and fatigue at these locations.

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable

FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

Not Applicable

SEQUENCE LISTING OR TABLES

Not Applicable

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to the reduction of stresses in the shell, bottom head, skirt and skirt base plate of pressure vessels and storage tanks. Shell configurations are typically cylindrical and may be vertical or horizontal in both tanks and pressure vessels, but may have alternatively shaped shells.

Both tanks and pressure vessels can have flat bottoms or heads featuring radii. If a vertical pressure vessel features a bottom head with a radius, typical, it will then most probably have a bottom skirt or jacket. This will typically be disposed to the shell near the head to shell joint. Most flat bottom tanks feature a perpendicular bottom to shell joint. However, when a flat bottom tank has a bottom to shell detail which features a knuckle (a section of a torus) then the shell, roof, shell appurtenances, and environmental loads must be considered when engineering this knuckle section. A skirt may be used to transmit these forces away from the knuckle section and into the foundation thereby reducing the knuckle thickness and costs associated with fabrication and erection.

If the tank or pressure vessel temperature or pressure varies the diameter will change. Typical anchoring of tanks and vessels is done through anchor chairs or a base plate attached to the bottom edge of the skirt. This results in the skirt base remaining fixed and unable to change in diameter as the vessel, and the top of the skirt, diameter changes. This deflection, as a result of vessel expansion, creates stresses in the skirt, shell, and skirt base plate. When this occurs as part of a scheduled process numerous cyclic operations occur over the life of the vessel. This cyclic expansion subjects the skirt, shell, and base plates and associated welds to cyclic ratcheting which may result in fatigue of the elements.

As the tank or vessel diameter, the change in temperature, and the number of temperature cycles are increased, and specifically coupled with a decrease in the skirt height, ratcheting becomes more of a problem due to the size and cost of the materials needed in this region. When expensive alloys are used this cost can be magnified due to required complex welding procedures.

2. Discussion of Prior Art

Not Applicable

3. Objects and Advantages

Accordingly, besides the objects and advantages of the Tank and Pressure Vessel Skirt Ratcheting Prevention Device as described in my above patent, several objects and advantages of the present invention are:

-   -   a. Reducing stresses on skirt, skirt base plate, and shell;     -   b. Reducing the thickness of the skirt, skirt base plate, and         shell;     -   c. Decreasing the cost of the skirt, skirt base plate, and         shell;     -   d. Increasing the life and reliability of the skirt, skirt base         plate, and shell;     -   e. Reducing the cost of storage tanks used in Cryogenic         applications and renewable energy molten salt storage.

SUMMARY

Placement of a pressure vessel or tank skirt at the base of a cylindrical shell disposed near the bottom head joint to said shell allows the self weight of the vessel and environmental loads on the vessel to be passed to a non-pressure member. When the tank or pressure vessel is subjected to large cyclic thermal or pressure changes, the diameter of the vessel will increase and decrease. With the skirt base plate resting on the foundation the base of the skirt is unable to change diameters as the skirt top diameter increases and decreases with the changing diameter of the vessel, resulting in ratcheting of the skirt. This ratcheting action of the skirt to shell and skirt to base plate results in a low fatigue life of these joints. Fatigue life is directly proportional to the dependability of the tank or vessel. The claims presented herein provide means for preventing this deflection and thus increase the life of the joints.

DRAWINGS—Figures

FIG. 1 shows the plan view of the Tank and Pressure Vessel Skirt Ratcheting Prevention Device featuring a ratcheting prevention member connecting the bottom and the skirt base plate, a ratchet prevention member stiffener, a key recess, and a key structure.

FIG. 2 shows a section view of the Tank and Pressure Vessel Skirt Ratcheting Prevention Device featuring a ratcheting prevention member connecting the bottom and the skirt base plate, a ratchet prevention member stiffener, a key recess, and a key structure.

FIG. 3 shows the plan view of the Tank and Pressure Vessel Skirt Ratcheting Prevention Device featuring a friction reduction device beneath the skirt base plate, a key recess, and a key structure.

FIG. 4 shows the section view of the Tank and Pressure Vessel Skirt Ratcheting Prevention Device featuring a friction reduction device beneath the skirt base plate, a key recess, and a key structure.

DRAWINGS—Reference Numerals

-   21 foundation -   22 bottom -   23 tank shell -   24 knuckle -   25 skirt -   26 skirt base plate -   27 ratchet prevention member -   28 stiffener -   29 tank or vessel -   30 friction reduction device -   31 key structure -   32 key recess

DETAILED DESCRIPTION—Preferred Embodiment—FIGS. 1 and 2

The preferred embodiment of the present invention, Tank and Pressure Vessel Skirt Ratcheting Prevention Device, is shown in FIG. 1, plan. Welded construction is the most common practice today but not the only method of joining metal plates. It is used here for its widespread industry acceptance. The tank or vessel 29 is shown placed on a typical foundation 21. The bottom 22 rests on the foundation 21 and is welded to the knuckle 24. The knuckle 24 is then welded to the tank shell 23 so as to provide a watertight pressure envelope. The upper end of the skirt 25 is welded to the tank shell 23 and skirt base plate 26 is welded on the lower end. The ratchet prevention member 27 is welded to the outside of the knuckle 24 and the skirt base plate 26. A stiffener 28 is welded to the ratchet prevention member 27. A key recess 32 in the foundation 21 and key structure 31 welded to the bottom of the tank shell 23 is best disposed near the center of the tank bottom 22.

OPERATION—FIGS. 1 and 2

The foundation 21 provides the support structure for the tank or vessel 29. The bottom 22, knuckle 24, and tank shell 23 make up the pressure boundary envelope of the tank or vessel 29. The skirt 25 reduces or completely removes the forces from the environment, tank structure and appurtenances from the knuckle 24. The skirt base plate 26 spreads these loads from the skirt 25 into the foundation 21. When the bottom 22 expands or contracts the ratchet prevention member 27 transfers the thermal expansion force on the knuckle 24 to the skirt base plate 26 keeping the top and the bottom skirt 25 diameters equal. As a result, stresses in the skirt 25, skirt base plate 26, and tank shell 23 are reduced. The stiffener 28 is used to provide buckling rigidity to the ratchet prevention member 27 when it is subjected to a compression load. As a result, ratcheting of the skirt base plate 26 is reduced or eliminated. The implementation of a key recess 32 and the properly spaced key structure 31 prevents excessive lateral displacements of the tank or vessel 29. Adequate clearance must be used between the key structure 31 protruding from the tank bottom 22 and the key recess 32 in the foundation 21 to accommodate any thermal expansion.

Description and Operation of Alternative Embodiments—FIGS. 3 and 4

The alternative embodiment for the reduction of stresses in the tank shell 23, knuckle 24, and skirt 25 of a tank or vessel 29 of the present invention, Tank and Pressure Vessel Skirt Ratcheting Prevention Device, is addressed using a friction reduction device 30. The tank or vessel 29 is shown placed on a typical foundation 21. The bottom 22 rests on the foundation 21 and is welded to the knuckle 24. The knuckle 24 is then welded to the tank shell 23 so as to provide a watertight pressure envelope. The upper end of the skirt 25 is then welded to the tank shell 23 and the lower end to the skirt base plate 26. The friction reduction device 30 is attached to the underside of the skirt base plate 26. This reduces friction under the skirt base plate 26 allowing the upper and lower diameters of the skirt to remain equal. As a result, ratcheting of the skirt base plate 26 is reduced or eliminated. The implementation of a key recess 32 in the foundation 21 and the properly spaced key structure 31 welded to the bottom of the tank shell 23, best disposed near the center of the tank bottom 22, prevents excessive lateral displacements of the tank or vessel 29. Adequate clearance must be used between the key structure 31 and the key recess 32 to accommodate any thermal expansion.

CONCLUSION, RAMIFICATIONS, AND SCOPE

Thus, it should be apparent that the skirt reduction of stress device described herein provides a reliable, yet economical solution to the process, natural gas, terminal, and renewable energy molten salt storage industries. Society and the renewable energy industry will benefit from a sound energy policy and lowered energy cost as a result of more reliable and cost effective LNG and molten salt storage systems. These two energy sources have enormous potential; Molten Salt for domestic base load power replacement and LNG for export and domestic use. 

I claim:
 1. A jacket ratchet prevention device with one edge contiguous to a storage tank shell comprising at least one structural element; so that one edge of said jacket is contiguous to the tank shell; such that the motion of the other edge of said jacket is slidably impeded due to friction; such that an axial and a horizontal force are urging said jacket; such that deflection of the jacket is prevented; such that the two jacket edge diameters are prevented from becoming unequal; such that said storage tank shell is the pressure boundary membrane of a containment device; whereby ratcheting of the jacket ends are prevented; whereby urging the jacket base plate end; such that said jacket edge diameters are equal; thereby reducing stresses in said jacket and said storage tank shell.
 2. The device in claim 1, wherein said forces are from the weight of the tank structure, a product contained in the tank shell, or from an environmental load.
 3. The device in claim 1, wherein an unattached edge of said jacket ratchet prevention device is contiguous to a jacket.
 4. The device in claim 1, wherein said jacket is a pressure vessel or tank skirt.
 5. The device in claim 1, wherein the shell to the ratchet prevention device connection is disposed near a transition from a cylindrical to a conical section.
 6. The device in claim 1, wherein the shell to the ratchet prevention device connection is disposed near a transition from a cylindrical section to a portion of a head.
 7. The device in claim 1, wherein said horizontal forces are a result of cyclic thermal expansion.
 8. The device in claim 1, wherein an unattached edge of said jacket ratchet prevention device is contiguous to the skirt base plate.
 9. A jacket ratchet prevention device contiguous to a storage tank skirt base plate comprising at least one antifriction device; so that one edge of said jacket is contiguous to the tank shell; such that the motion of the other edge of said jacket is slidably impeded due to friction; such that an axial and a horizontal force are urging said jacket; such that deflection of the jacket is reduced; such that said storage tank shell is the pressure boundary membrane of a containment device; whereby reducing the difference of the two jacket edge diameters; whereby ratcheting of the jacket ends is reduced; whereby reducing the resisting force urging the sliding end of the jacket; such that said jacket edge diameters are closer; thereby reducing stresses in said jacket and said shell.
 10. The device in claim 9, wherein said forces are from the weight of the tank structure, a product contained in the tank shell, or from an environmental load.
 11. The device in claim 9, wherein said horizontal forces are a result of cyclic thermal expansion.
 12. A tank drift device so as to prevent a tank or vessel from sliding due to lateral urging comprising: a. a structure contiguous to the underside of the bottom of the tank; such that said tank drift device is disposed near the center of the tank and b. a reinforcing structure to effectively transmit urging forces from said structure to the tank and c. an adequately reinforced, keyed recess in a tank foundation; such that said tank drift device structure fits into the keyed foundation, whereby, movement of the tank at the perimeter is greatly reduced.
 13. The device of claim 12, wherein said tank drift device is a bottom sump.
 14. The device of claim 12, wherein said tank drift device is a conical bottom. 